1. Field of the Invention
This invention relates to a solid-state imaging device for imaging an object. More particularly, it relates to a solid-state producing an image signal of device in which adjustment of light exposure may be made automatically.
2. Description of Related Art
In a video camera, for example, there is known, as a mechanism for automatically adjusting the volume of light received by a so-called CCD image sensor constituted by photoelectric converting elements as solid-state imaging elements and charge-coupled devices, a mechanism for automatically adjusting a so-called iris enclosed in a lens part of the camera, referred to hereinafter as an auto-iris mechanism, as disclosed in, for example, JP Patent Publication Kokai No. 63-82067 (1988).
Referring to FIG. 5, the video camera is constituted by a lens part 50 and a main body of the video camera 60.
The lens part 50 is constituted by a lens 51, an iris 51, a detection circuit 53 for detecting the level of imaging signals transmitted from the main body of the video camera 60, a comparator circuit 54 for comparing an output of the detecting circuit 53 with a reference voltage, and an iris driving circuit 55 for controlling the opening and closure of the iris 52 based on an output of the comparator circuit 54.
The main body of the video camera 60 is constituted by a CCD image sensor 61, made up of solid-state imaging devices, an amplifier 62 for amplifying imaging signals from the CCD image sensor 61, an automatic gain control circuit (AGC circuit) 63 for processing the amplified imaging signals from the amplifier 62 by automatic gain control, and a signal processing circuit 64 for converting the imaging signals from the AGC circuit 63 into video signals conforming to the NTSC or PAL system.
The auto-iris mechanism is so constructed that an output level of the CCD image sensor 61 accommodated in the main body of the video camera 60 is fed back to the iris 52 accommodated in the lens part 50.
That is, the opening and closure of the iris 52 is adjusted automatically so that an output level of the CCD image sensor 61, obtained by means of the amplifier 62 and the detection circuit 53, will be the reference voltage or so that the output of the comparator circuit 54 will be equal to zero.
On the other hand, as a light exposure adjustment mechanism not making use of an iris, there is also known a mechanism in which the charge accumulation time of a so-called charge accumulating type CCD image sensor is controlled, referred to hereinafter as an electronic shutter.
More specifically, with the field accumulation type CCD image sensor having the function of the electronic shutter, an image readout high-level pulse SG shown in FIG. 6B is issued when a low level signal indicating a vertical blanking period as shown in FIG. 6A, referred to hereinafter as a vertical blanking signal, is supplied, and charges accumulated since the issuance of the image readout pulse SG for a given field until issuance of the image readout pulse for the next field are read out on the basis of the image readout pulse SG of the next field.
Referring to the function of the electronic shutter, high-level reset pulses SUB are supplied to a substrate of the CCD image sensor during the horizontal scanning period since the time of issuance of the image readout pulse SG of a given field, as shown at FIG. 6C, to drain off the charges accumulated until then, and the time period since the issuance of the last reset pulse SUB until the issuance of the image readout pulse SG of the next field is controlled to control the charge accumulating time T.sub.CHG. In the NTSC system, for example, the maximum charge accumulating time T.sub.CHG is 16.7 ms as determined by the field frequency, whereas, in the PAL system, the maximum charge accumulating time T.sub.CHG is 20 ms as determined by the field frequency.
Meanwhile, in an industrial video camera, for example, a number of exchange lenses by the so-called C-mount system are used, such that the main body of the video camera may be used in combination with freely selected lenses. However, with an auto iris lens system making use of the above mentioned auto iris mechanism, various problems are presented in connection with interfacing between the lens system and the main body of the video camera. These problems include that associated with interchangeability of a connector interconnecting the auto iris lens system and the main body of the video camera and that associated with the matching of the standards for the feedback signal level, current capacity and the source voltage supplied from the main body of the camera to the auto iris lens system.
Also, since the detection circuit 53, the comparator circuit 54 etc. are accommodated in the lens part 50, as shown in FIG. 5, it is necessary to adjust the reference voltage etc. in the lens part 50 each time the lens part 50 is exchanged to provide for optimum light exposure.
On the other hand, the auto iris lens system is generally expensive and involves complicated cable connection as compared with a manual iris lens system in which the iris is adjusted manually.
In addition, in the adjustment of the light exposure time making use of the above mentioned electronic shutter, the reset pulses sub draining the accumulated charges need to be applied during the horizontal blanking period to prevent any adverse effects on the currently read out imaging signals. For this reason, the charge accumulating time T.sub.CHG is controlled with the 1 H time period, that is, 64 .mu.s, corresponding to a period of the horizontal scanning period, as a unit. Therefore, while no problem exists with 1 H timing resolution control of the charge accumulating time T.sub.CHG for a lower shutter speed range corresponding to a dark object, the 1 H timing resolution control is too coarse for practical use for a higher shutter speed range corresponding to a bright object.